A range of spring loaded poppet valves designed to automatically stop an engine by closing down the air intake should excessive overspeeding occur. These valves are available with standard springs or strong springs (indicated by the suffix “S” after the valve type number). The spring selection is determined by the rating of the engine to which the valve is to be fitted.
The closing force on the valve is provided by the intake air flow passing through. As the air flow increases, the closing force builds up. This is resisted by the valve springs force, the pre-load of which is adjustable such that at a given airflow the resulting force overcomes the spring resistance and causes the valve to close. Once closed the valve will not reset to the open condition until the engine stops.
This type of valve may be fitted to either naturally aspirated or turbocharged engines. It should be noted however that for a given valve setting the repeatability of the actual automatic shut down speed has a greater scatter in the case of a turbocharged engine. However, unless for special reasons a precisely repeatable shutdown speed is required, adequate protection from excessive overspeed and potential resulting damage is still achieved.